Authors:

Paulo Arratia(University of Pennsylvania)

Boyang Qin(University of Pennsylvania)

Ranjiangshang Ran(University of Pennsylvania)

Paul Salipante(NIST)

Steven Hudson(NIST)

Over a decade ago Jerry Gollub along with two talented Haverford College
undergraduate students and one of the authors (PEA) investigated the flow of
viscoelastic fluids near hyperbolic points using a cross-slot channel. We
found two novel flow instabilities of a planar extensional flow under steady
forcing: (i) a symmetry breaking and steady instability and (ii) a
time-dependent flow as strain-rate is increased. Here, new experiments using
3D particle tracking velocimetry are presented. We find a new
symmetry-breaking instability \textit{normal} to the extension plane, marked by a bi-stable
flow switching in that direction and ``buckling'' of the separatrix between
the impinging inflow streams. A strong connection exists therefore between
temporal fluctuations and 3D flow. We explore this connection in some detail
comparing the distribution of 3D modes with fluctuations of local pressure
in inlet channels